Call progress tones are signals in the audible frequency band which are provided to a telephone caller during the progress of setting up, holding or lo transferring the call, such as busy tone, ring back tone, dial tone, etc. Call progress tones consist single frequency and dual-frequency combinations sinusoidal voltages which are applied in specific cadences, e.g. the length of time the tone is on or of and its repetition pattern.
The frequency components of a call progress tone are identified by its numerical frequency, measured in Hz, a frequency tolerance (.+-.Hz) and intensity level (dBm). Speech signals are characterized by irregular bursts of combinations of many frequency components, and have non-specific on and of patterns.
Various countries use different call progress tone standards and with cadencing plans different from each other. It is important for a telephone switching system that is to be sold in various countries to have a programmable call progress tone detector in order to easily configure the switching system to the needs the particular country in which the system is to be sold.